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' • 15 October 2002 AGE-NC Project No 96-0232 ' Page 17 of 19 During the September 2002 sampling event, subtle to moderate decreases in MTBE concentrations ' were reported from wells MW-7,MW-8A,MW-9A,MW-11A,MW-1113,EW-1,EW-4,EW-8 and EW-10 The data implies that the decreases may be due to the overall MTBE mass reduction in First ' Zone water and possibly Second Zone (MW-9A, MW-11A) and Upper Rocky's Zone water (MW-11B) ' Analytical results from the September 2002 quarterly monitoring event indicates the most intense on-site MTBE impact is in the First Zone near EW-5 (24 µg11), EW-9 (43 µg/1) and in the Second Zone near MW-8A(8 6 gg/1),the most intense off site MTBE impact is in the First Zone near MW-7 ' and Three B's Truck Plaza well MW-4(22 gg/l and 56µg11,respectively),and in the Rocky's Upper Zone well MW-11 B(79 gg/l) It is not clear at this time why the Three B's Truck Plaza well MW-4 has consistently had greater MTBE-impact than that of intermediate well MW-7 ' Figures 11 and 12 depict the approximate extent of MTBE-impacted ground water beneath the site in cross sectional views C-C' and D-D', Figures 8, 9 and 10 depict the estimated extent dissolved MTBE-impact in plan view These figures depict Flag City Chevron wells EW-1 through EW-10, MW-7,MW-8A,MW-11B, MW-11D and Three B's Truck Plaza well MW-4 to be located within dissolved MTBE-impacted plumes The vertical extent of impacted ground water on-site appears to lie between 17 feet bsg(estimated depth to ground water)and 35 feet bsg(Figures 11 and 12) Low concentrations of MTBE were previously detected in the Second Zone Aquifer (MW-9A) and Primary Aquifer (MW-9C), but were not-detected in water samples for MW-9A during the ' September 2002 sampling event and have been non-detect at MW-9C for four consecutive sampling events ' Going off-site toward the west,the plume may extend to as much as 75 feet bsg approaching the area of MW-11 B,near the former Rocky's Restaurant domestic well(RDW-1)where the underlying fine- grained sedimentaryunits may thin and pinch out, low concentrations of MTBE have been detected in Primary Aquifer (MW-1 ID), screened at between 130 and 150 feet bsg Further westward from MW-11, the plume of water impacted by MTBE appears to extend beyond Three B's Truck Plaza well MW-4, with a vertical extent from possibly 17 feet bsg to possibly 75 feet bsg The full westward extent of the Flag City Chevron plume is uncertain at this time Previous contaminants encountered in First Zone ground water at MW-13 include TPH-d and TPH-mo which suggest that ' the contaminants may be from a source other than the Flag City plume It is not known if the suspected plumes are co-mingled in the area between Three B's Truck Plaza wells MW-1 through MW-6 and Flag City Chevron wells MW-7,MW-11,MW-12 and MW-13,AGE has modeled them ' as separate plumes (Figure 8) Relative to the inferred source area near the Flag City Chevron UST area (EW-1), the Flag City Chevron plume appears to have extended approximately 200 feet to the west at depths as great as • 75 feet bsg,however,AGE suspects the plume may have pinched and/or separated between Thornton Road (Figures 8 and 11) The plume extends approximately 30 feet to the south at depths as great Advanced GeoEnvironmental,Inc 1